Two unique macrolactones were detected during mass screening for Taxol-like substances by both Merck Research Labs and the GBF (Gesellschaft fur Biotechnologische Forshung mbH, Germany). Bollag, D. M., et al., Epothilones, a new class of microtubule-stabilizing agents with a taxol-like mechanism of action. Cancer Res., 1995. 55(11): p. 2325-33; Bollag, D. M., Epothilones: novel microtubule-stabilizing agents. Expert Opin. Invest. Drugs, 1997. 6(7): p. 867-873. As a result, Epothilone B 1, and Epothilone A, 2, were both isolated from the myxobacterium Sorangium cellulosium and the two dimensional structures were determined by the Merck group using NMR spectroscopic methods (HMBC), and the X-ray structure was published by the GBF group. Höefle, G., et al., Antibiotics from gliding bacteria. 77. Epothilone A and B—novel 16-membered macrolides with cytotoxic activity: isolation, crystal structure, and conformation in solution. Chem. 1996, 108, 1671-1673; Angew. Chem., Int. Ed. Engl., 1996. 35(13/14): p. 1567-1569. Both macrolides 1 and 2 appear to possess identical modes of action to Taxol, but are thousand-fold more potent in multidrug resistant cell lines. Taxol 3 is perhaps one of the most structurally complex anticancer agents known. Since its discovery in the early 1970's, it has received great attention from the scientific and medical community. Borman, S., Scientists Mobilize To Increase Supply of Anticancer Drug Taxol, in Chem. & Engr. News. 1991. p. 11-18. Taxol binds to the microtubule, or polymeric, form of tubulin with micromolar KD values and stabilizes the microtubule pool by blocking the transition between G2 and M phases of cell growth. Taxol has a specific binding site in the polymeric tubulin. While other agents which arrest mitosis are known and are in clinical use for cancer chemotherapy, Taxol has elicited much attention for its efficacy against drug-refractory tumors, most notably ovarian but also metastatic breast, head and neck, melanoma and lung cancers. Taxol has recently been approved by the Food and Drug Administration for treatment of ovarian cancer (1992), breast cancer (1994) and is expected to be approved for other cancers. Riondel, J., et al., Cancer Chemother. Pharmacol., 1986. 17: p. 137. Thus, while Taxol finds clinical promise against refractory cancers, substantial problems are none the less associated with this anticancer agent. Taxol is only poorly water soluble necessitating its administration in Chremophor, a solvent that in itself can be more toxic than Taxol and has caused a number of clinical problems. Further, more serious complications include peripheral neuropathy, neutropenia, cardiac arrythmias, and less problematical, alopecia. Perhaps not surprisingly, Taxol is itself a genetic toxin at levels comparable to those in clinical use. Finally, perhaps some of the toxicity issues are related to the short plasma half-life of Taxol (less than 5 hrs). Kumar, G., T. Walle, and U. Walle, Cytochrome P450 3A-Mediated Human Liver Microsomal Taxol 6a-Hydroxylation. J. Pharmacol. Exp. Ther., 1994. 268; p. 1160-1165.
The most interesting feature of Epothilone B is that it behaves essentially identically to Taxol 3 in vitro, yet is thousand-fold more active than Taxol in cancerous cells which have acquired multiple drug resistance (MDe), has the advantage of better solubility than taxol, and can be obtained in multigram quantities. Bollag, D. M., et al., Epothilones, a new class of microtubule-stabilizing agents with a taxol-like mechanism of action. Cancer Res., 1995. 55(11): p. 2325-33; Bollag, D. M., Epothilones: novel microtubule-stabilizing agents. Expert Opin. Invest. Drugs, 1997. 6(7): p. 867-873; Buck, S. B., et al., Epothilones, a new class of microtubule-stabilizing agents with a taxol-like mechanism of action. Chemtracts, 1998. 11(9): p. 671-677; Grever, M. R.; Schepartz, S. A.; Chabner, B. A. Seminars in Oncology 1992, 19, 622-638. In P-glycoprotein (the MDR protein which pumps drugs out of the cell) expressing KBV-1 cells, for example, IC50 values for Taxol are 2.3×10−5 M but are 5.8×10−8 M for Epothilone B. These effects seem to be expressed on a mechanistic level; Epothilone B binds competitively with Taxol to the Taxol binding site but is presumably a much poorer P-glycoprotein substrate. Since the most remarkable feature of Taxol is its good activity against MDR cancers, and Epothilone B is far superior to Taxol in this regard, it is likely that Epothilone B will evolve to have a much greater therapeutic index than Taxol against MDR cancers. At the very least, Epothilone B (analogs) would be a useful next line of clinical chemotherapy once Taxol resistance had been encountered. While much more research remains to be done for the epothilones, it still seems as if Epothilone B could well become an anticancer drug that is clinically far superior to Taxol.
Epothilone B has one main ring, a 16-membered lactone ring with a total of 7 stereocenters. In comparison, Taxol has four main rings and 11 stereocenters and has occupied the best minds in synthetic organic chemistry for at least the last ten years with no less than 30 groups working on its total synthesis at one time or the other over this period of time. While its total synthesis was completed by Holton, Danishevsky and Nicolaou (Holton, R. A., et al., First Total Synthesis of Taxol. 1. Functionalization of the B Ring. J. Amer. Chem. Soc., 1994. 116: p. 1597-1598), the incredible complexity of Taxol has hampered the development of a viable total synthetic route by which Taxol or its analogs could be obtained for clinical use. However, the supply issue for Taxol was solved to some extent by partial synthesis from baccatins, available from ornamental yew plants. Ojima, I., et al., New and Efficient Approaches to the Semisynthesis of Taxol and Its C-13 Side Chain Analogs by Means of β-Lactam Synthon Method. Tetrahedron, 1992. 48: p. 6985-7012.
The synthesis of Epothilone B, on the other hand, should not represent an insurmountable task. Epothilone B has yet to receive the attention that Taxol has, but is in principle inexhaustibly available from fermentation. However, practical experience has led the Merck group to abandon the preparation of Epothilones by fermentation due to extremely poor yields, a finding confirmed by NaPro Biotherapeutics. Thus, in order to obtain Epothilone B for clinical trials, semi-synthetic modification, structure-activity relationship studies, and to make it commercially available, an efficient total synthesis is required.
Several excellent syntheses of Epothilone A and B have appeared in the last few years, as reported in Appendino, G. and G. Casiraghi, The synthesis of epothilones: highlights from a year's race. Chemtracts, 1998. 11(9): p. 678-696. Numerous reported syntheses and partial syntheses, as well as patent-related publications have also appeared in the past few years in regards to Epothilone syntheses: Nicolaou, K. C.; Roschangar, F.; Vourloumis, D. Angew. Chem. 1998, 110, 2121-2153; Angew. Chem. Int. Ed. Engl. 1998, 37, 2014-2045; Mulzer, J. Chem. Mon. 2000, 131, 205-238; Meng, D.; Bertinato, P.; Balog, A.; Su, D.-S.; Kamenecka, T.; Sorensen, E.; Danishefsky, S. J. J. Am. Chem. Soc. 1997, 119, 100073-10092; Nicolaou, K. C.; He. Y.; Vourloumis, D.; Vallberg, H.; Roschanger, F.; Sarabia, F.; Ninkovic, S.; Yang, Z.; Trujillo, J. I. J. Am. Chem. Soc. 1997, 119, 7960-7973; Nicolaou, K. C.; Ninkovic, S.; Sarabia, F.; Vourloumis, D.; He. Y.; Vallberg, H.; Finlay, M. R. V.; Yang, Z. J. Am. Chem. 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However, there still remains a need for approaches that will provide an efficient route to Epothilone A, B or Deoxyepothilone A or B, or to analogs and derivatives of these compounds.